The present invention relates to a steering wheel for a motor vehicle with a driver airbag module, in which the module snaps into an aligned engagement with the steering wheel during manufacture of the steering wheel.
Motor vehicle driver airbags are normally located in the center of the steering wheel, and often in an airbag module which may be pressed to activate the vehicle horn. Such modules float on a spring biasing means sufficiently soft to be activated by a user""s finger. The modules also need to be retained to a supporting portion of the steering wheel, for example a metal frame in such a way that the high forces exerted during inflation of the air bag, owing to contact of the inflating air bag with the surrounding steering wheel, do not cause the module to be pulled off the steering wheel.
It is also generally desirable that such airbag modules are relatively inexpensive to manufacture and can be quickly and inexpensively affixed to the steering wheel during manufacture of the motor vehicle.
A number of arrangements have been proposed for such steering wheels. One prior art steering wheel with an airbag is disclosed in EP 0 754 603 A1, in which a pair of cylindrical legs fix the module to a steering wheel frame. The frame has apertures with clearance for the legs, and each leg has four tabs or barbs that are equally spaced around the circumference of the leg. The barbs are depressed against the surface of the leg during insertion of the legs through the apertures, and squeeze through the aperture to spring outwards beyond the edges of the aperture to keep the module retained to the steering wheel frame.
A problem has been noted with such prior art steering wheels. Although the airbag module may be quickly snap-fitted to the frame during manufacture of the motor vehicle, the need for clearance between the leg and the aperture to allow the legs and barbs to pass through the apertures, means that there is transverse slack in the attachment of the module to the steering wheel frame. This slack has a number of undesirable consequences. The airbag module will often be covered in a plastic material cover, surrounded by a similar plastic covering the rest of the steering wheel. In the case where the horn is activated by pressing the module, there is a need to prevent the module from interfering with the surround, and the transverse slack means that the gap between the module and the steering wheel surround may need to be as great as 2 mm, which may be unsightly. Another problem is that the module may rattle transversely owing to vibrations transmitted up though the steering column.
According to the invention, there is provided a steering wheel airbag assembly for a motor vehicle, comprising: an airbag supporting portion with a plurality of apertures therein; a driver airbag module attached to the supporting portion and having a plurality of legs, each of the legs extending into a corresponding aperture and being slidable therein so that the module has a degree of travel towards and away from the supporting portion; engagement means provided between each leg and aperture to limit said degree of travel and keep the module attached to the supporting portion; a spring biasing means between the module and the supporting portion by which the module is spring biased away from the supporting portion, wherein a leg has a first pair of outwardly opposed surfaces that form a loose sliding fit with the aperture, and a second pair of outwardly opposed surfaces that form a tight sliding fit with corresponding surfaces of said aperture in order to locate the module in one transverse direction on the supporting portion as the module is moved towards and away from the supporting portion.
The tight sliding fit allows the airbag module to move longitudinally in a direction defined by the legs, as may be needed, for example in order to make a connection to activate a horn or some other device in the motor vehicle.
In a preferred embodiment of the invention, the loose sliding fit of the leg defines a slack axis of the aperture, the slack axis being oriented generally towards a central portion of the steering wheel. The slack between the leg and aperture can therefore take up dimensional variation between the module and supporting portion, which will be more apparent across the extent of the module and support assembly than over small distances.
The engagement means may be one or more barbs towards the end of a leg which engage with one or more corresponding edges of an aperture.
In a preferred embodiment of the invention, a leg has four sides, for example with a square or a rectangular cross-section, the first and second outwardly opposed surfaces being provided by the four sides and the barb means being provided on at least one of the sides. For reasons of strength, barbs should preferably be on both of an opposite pair of sides.
Preferably, a pair of barbs is provided one on each of the second pair of outwardly opposed surfaces. Because this pair of surfaces makes a tight sliding fit with the aperture, there is no easy way for the barbs to slip back into the aperture and so disengage the module from the supporting portion. The module is therefore securely attached to the supporting portion.
The steering wheel may be designed so that the module can be assembled to the supporting portion in a press-fit operation in which the engagement means snaps into engagement to attach the module to the supporting portion. This avoids the need for an assembly worker to get behind the steering wheel, for example to secure the module with bolts. The arrangement may be such that the xe2x80x9csnapxe2x80x9d is clearly audible, so providing a positive confirmation that the module is indeed secured to the supporting portion.
To aid insertion of the legs into the apertures during assembly of the steering wheel, the end of one or more of the legs may be tapered to aid insertion of said leg into the apertures. The engagement means may then be such that this prevents further insertion of said leg until a sufficient pressure is bough to bear to complete the insertion and snap the engagement means into engagement. Because a steering wheel airbag module is normally flush with the surrounding steering wheel, this provides the benefit that if a worker does not complete the assembly by failing to press-fit the module to the supporting portion, then the module will stand proud of its surrounds, thereby providing an obvious visual cue that the airbag module is not corrected seated in the steering wheel.
Advantageously, the degree of travel permitted by the tight sliding fit of the legs in the apertures allows over-insertion of a leg into an aperture during assembly of the module to the supporting portion to ensure that the engagement means such as barbs are properly engaged.
The transverse location provided by close contact between the second pair of surfaces and the aperture will tend to increase the accuracy of the transverse location of the module with respect to the rest of the steering wheel, so allowing improvements to be made in the way the module fits in the steering wheel. For example, the improvement may allow a gap between the module and a surround to be reduced from 2 mm to about 1 mm or less.
Preferably, there are three or more of the legs, at least one of which has first and second pairs of outwardly opposed surfaces angled to corresponding pairs on the other legs so that the tight sliding fit of the second pairs of outwardly opposed surfaces of said three or more legs locates the module on the supporting portion in orthogonal transverse directions. For example, there may be three such legs, two of which have outwardly opposed surfaces similarly orientated with respect to each other, the third leg then being angled at 90xc2x0 to the other two legs. The pair of similarly oriented legs therefore each locate the module in one transverse direction, whilst the third leg orients the module in an orthogonal transverse direction, and together all three legs prevent rotation of the module about a longitudinal axis passing between all three of the legs.
In general, the tight sliding fit may result in some stiction that tends to prevent the module moving away from the supporting portion. Therefore, the spring biasing means may advantageously provide a spring bias that is sufficiently strong to overcome this stiction.
One way of providing the spring biasing means is to provide a coil spring wrapped around one or more of the legs. Each leg therefore retains the coil spring in place.
The steering wheel will generally have a pair of electrical contacts for a horn. One of the contacts may be provided on the module with the other of the contacts being provided on the supporting portion. The contacts then are closed when the module is moved towards the supporting portion.
Optionally, the supporting portion may comprise a horn plate. The horn plate need not provide structural strength, but may be disposed between a structural frame of the supporting portion and the module. Then one of the contacts may be provided on the module with the other of the contacts being provided on the horn plate, again the contacts being closed when the module is moved relatively towards the supporting portion.
The horn plate may have a number of apertures through which the legs pass when the module is attached to the supporting portion. Because the module will be located with respect to apertures in the supporting portion frame, there is no need for the horn plate also to have apertures. However, if the horn plate has apertures, each of these may have clearance for the free movement of the legs relative to the horn plate when the module is moved towards and away from the supporting portion.
One way in which the tight sliding fit may be provided is if the dimensional tolerances of the legs, apertures and distances between the legs and apertures is closely controlled, In practice however, this may be difficult to achieve at a reasonable cost. Therefore, it is preferred if one or more of the legs are compressible at least in a direction normal to the second pair of outwardly opposed surfaces. The distance between the second pair of surfaces may then be greater than the separation between the corresponding aperture surfaces so that the tight sliding fit is provided by the inward compression of the second pair of surfaces by the corresponding aperture surfaces.
When engagement means such as barbs are on the second pair of surfaces, the combination of a tight sliding fit and a snap engagement of the legs in the apertures is helped by the fact that the legs may be compressible. It is therefore not necessary for the legs to have a recess to allow the barbs to be pressed flat against the surface of the legs when the barbs are inserted trough the supporting portion apertures as the module is assembled to the supporting portion.
One way in which the legs may be made compressible, is if a leg has walls formed of a resiliently flexible material around a hollow center. Another way is if the legs are formed from plastic coated thin sheet steel, which may have a U-shaped cross-section around a hollow center. Steel in particular has a high tensile strength, which is useful in keeping the module secured to the supporting portion, even when stressed owing to the high forces present tending to pull the airbag module away from the supporting portion when the airbag is inflated.
One way in which a smooth tight sliding fit may be achieved is if the second pair of outwardly opposed surfaces are of a plastic material and the corresponding surfaces of the aperture are metal. The plastic material may, for example, be a nylon plastic. It has been found that the fit between the plastic material will tend to improve the first few times the module is moved longitudinally, owing to a burnishing effect of the plastic by the harder metal. The movement therefore becomes smoother with use, for example after the first few times a horn has been activated. The burnishing effect naturally leads to a close, tight fit, and this together with the use of a slippery plastic material tends to eliminate transverse slack whilst at the same time allowing for squeak and rattle free operation.